1. Field of the Invention
This invention relates to a thermally insulating product of a silica-hydrate material and the process for foaming this product. More specifically, this invention discloses a thermal insulating material of expanded silica expanded or foamed into discrete bubble structures, which bubbles agglomerate to define a macrospecimen having improved thermal insulating characteristics. These foams are rigid, fire-resistant and vermin resistant. The foamed or expanded material may be made into shapes such as sheet products, utilized as a loose fill or as a sprayed-on application for structural insulating components.
2. Prior Art
Expanded or foamed silica-hydrate products are known in the art. Many of these known foams were produced utilizing nucleating agents, glass crystals, binders, and preconditioning of the raw material such as by selection of particle size. The processes for the formation of such foamed material generally include the use of high temperatures, a requirement of an alkali silica-hydrate, or a dehydration stage. Rapid heating of the raw material produces a foam product that includes a shell-like material or crust about the foamed silica. The shell is formed by more rapid dehydration of the silica-hydrate particle surface than is experienced in the interior of the particle. In addition, indiscriminate rapid heating of the raw material foams the particle outer portion which insulates the internal particle against heat transfer and subsequent expansion.
The use of additives as nucleating or cellulating agents to enhance the foaming process has been disclosed, as well as the use of particle additives for strengthening of the foamed product.
U.S. Pat. No. 3,532,480--D'Eustachio teaches a process for the manufacture of a cellular glass. The process includes the use of a pulverized glass and a cellulating agent, which are formed into pellets or some other preprocessed shape. These preformed pellets are heated to an elevated temperature which adhere to each other and form a sheet of coalesced expanded pellets or nodules. The expanded or coalesced pellets are fed into a second heating chamber for further cellulation and formation of a sheet of multi-cellular glass. The preheating or pelletizing operation is conducted at a temperature between 1500.degree. F. and 2200.degree. F. The foaming or second step of the process is performed on a molten metal bath which is preferably tin or a tin alloy. However, it is noted that any molten state metal or alloy that will serve as a suitable heat exchange media without adherence to the molten glass may be utilized. In addition, an inert or reducing gas atmosphere is utilized to maintain a substantially non-oxidizing atmosphere above the surface of the molten metal bath.
U.S. Pat. No. 3,756,839--Rao discloses forming of an alkali metal silicate to produce rigid foams suitable for use as insulation, which foams have low solubility, low density and high compressive strength. Rao U.S. Pat. No. 3,756,839 taught the following: (1) that finer particle size appears to result in the production of smaller and more numerous gas bubbles which inferentially implies finer bubbles, thinner walls, and smaller diameter bubbles; (2) that slow heating tends to dehydrate the silica-hydrate resulting in reduced expansion efficiency unless heating is done under saturation conditions approaching 100% relative humidity; and (3) that a fixed sample would tend to foam on its outer portion thereby insulating the inner area from the heat source. This disclosure teaches the introduction of an exothermic metal powder as a nucleating agent to improve the foaming step by enhancing the volatilization of the water in the mixture and the uniforming of the foaming action. This patent particularly calls for the addition of the use of alkali metal silicate particles of a 1 to 400 micron range and the use of a finely divided metal powder in the range of 1 to 100 microns, which finely divided powder is to be utilized as centers for nucleation. The alkali metal silicate is hydrated and subsequently thermally foamed with the use of external energy. The forming utilizes water evolved at the nucleating site as the blowing agent. This patent particularly teaches away from the use of loose or loosely compacted materials and notes the probability of a nucleator, that is one of the small micron size particles, projecting through a cell, that is bubble wall, which could lead to faster permeation of heat through the cell wall or could provide a point location for wall breakdown and premature cell failure.
A cellulated silica with a uniform distribution of closed cells produced by heating silica in the presence of a carbonaceous cellulating agent is taught in U.S. Pat. No. 2,890,126. The mixture of silica and cellulating agent is heated to a sintering temperature for the silica particles. The temperature is further raised to react the entrapped or suspended cellulating agent with the silica to produce entrapped gases which expand to form closed cells of small but uniform size. Further, the silica and cellulating agent must be finely ground and intimately mixed, such as by blending in a ball mill. The operating temperatures for the sintering and elevated temperature operations are about 2600.degree. F. and 2950.degree. F., respectively. This is a high temperature operation utilizing a cellulating or nucleating agent to provide the gas bubble requisite for the production of a foamed silica material.
U.S. Pat. No. 3,365,315--Beck et all teaches the formation of glass bubbles through a process for the direct conversion of glass cullet particles into glass bubbles by heating. The particular glass bubbles of this disclosure are for an improved or elevated strength application particularly useful in molded parts for high pressure environments. Beck U.S. Pat. No. 3,365,315 also indicates such particles may be designed with thin walls and limited crushing strength. The glass bubble diameter may vary from a few microns, Beck suggests between 5 and 10 microns as the lower limit, up to approximately 100 or even 300 microns. The relative wall thickness of the glass balloon is indicated as being from a fraction of a micron up to approximately 10% of the diameter of a complete glass bubble, that is 20 microns thick. This is a high temperature process for a glass having a melting temperature between approximately 1200.degree. C. and 1500.degree. C. The raw material fusion process is performed in an oxidizing atmosphere. The fused glass particles are reheated under a neutral or reducing atmosphere for maximum glass bubble yield. The product is a bubble whose diameter is between 24 and 67 microns, which bubble has an average wall thickness of 1.8 microns with a range of 1.2 to 2.2 microns. This is a high temperature bath operation requiring quenching with a water spray and a recycle process to achieve the 24 to 67 micron range bubble.
U.S. Pat. No. 3,466,221--Sams et al discloses the reinforcement of a foamed alkali silicate with inorganic fibers such as asbestos fibers. The alkali silicate is expanded by the reaction of finely divided silicon with the silicate. The blend of silicon and slightly moist silicate is allowed to expand until set at an optimum expansion by the addition of sodium fluosilicate. This is essentially a low temperature operation wherein the operating temperature may be as high as 90.degree. C. A microwave oven is used for product drying after formation of the foam.
U.S. Pat. No. 3,151,966--Slayter discloses a glass foam and the formation thereof. The desirability of a foam glass insulation with an apparent density below about 3 pounds per cubic foot and maintenance of individual, unconnected bubble units such that the foaming gas enhances the insulating characteristics of the glass foam is noted. The broad process steps of this particular patent teach the preparation of a melt at the range of 2460.degree. F. to 2550.degree. F. under oxidizing conditions; the bubbling of a heavy gas (e.g. sulfur dioxide) through the glass melt until fully saturated; thereafter, cooling and exposing the melt to shortwave radiation for crystal nucleation; reheating the melt to about 970.degree. F. for further rapid crystal nucleation; and, thereafter heating the glass above its softening point for crystal formation therein. This crystal forming temperature can be as low as 1200.degree. F. This again is a high temperature operation utilizing radiation or microwave energy only for brief periods after elevated temperature operations. The radiated melt temperature is elevated to a higher temperature. The product has multiphase glass structure in the walls of the bubbles or cells, and the process requires a nucleating agent and crystal formation above 1200.degree. F. There is some indication of a glass matrix with crystals embedded therein, which may be indicative of a shell or crust like structure inherent in these foam silicate products.
U.S. Pat. No. 3,743,601--Rao teaches the production of a microcellular inorganic silicate foam for building construction. The process includes steps of hydrating finely divided particulate silicates with water and thereafter expanding the mass by input of thermal energy. The heat energy may be provided by conventional heat sources, a microwave energy source or the dissipation of mechanical energy within the material. The hydrated material includes a solid alkali metal silicate and the hydrated water acts as the foaming agent. A further embodiment of the process includes the use of superatmospheric pressure as a process requirement. The resultant product of this material can provide a product with an R-rating up to R-5, however, the density ranges between 2 and 8 pounds per cubic foot.
U.S. Pat. No. 3,870,496--Cutler discloses a method for the production of a foam glass from materials such as waste glass. The glass is modified with hydroxide group materials and heated to the softening range of the glass composition to release the hydroxide, thereby foaming the glass. Water vapor is trapped in the stiffened glass forming a stiffened cellular product. Indicative of this reaction is the experiment wherein glass beads were hydrated in an autoclave with a saturated water vapor atmoshpere at 390.degree. C. and 1090 psi. The modified glass was treated in a furnace at approximately 1100.degree. C. to foam the material as well as partially sintering the foamed glass sample. Sintering would lead to a continuum on the surface of the foamed glass. Finely ground glass, that is finely divided particles, is required for an autoclaving process necessary to saturate the glass structure with hydroxide ions and the subsequent passage through a furnace for foaming.
A process for the production of a foamed alkali metal silicate is taught in U.S. Pat. No. 4,080,187--Parnell. The basic process includes the following steps: (1) hydration of an anhydrous glass composition form among the group of alkali metal glasses; (2) dehydration of the hydrated glass by about 50%; (3) rehydrating the glass foamed particles to an the integral mass without collapse thereof; and (4) dehydrating the integral mass to foam the silica-hydrate. Step 3 of the process is performed at or below 250.degree. C. but above 100.degree. C. A metal oxide is added to aid in reducing the surface tension of the glass melt.
U.S. Pat. No. 4,059,425 to Brydges, III et al, discloses a process and apparatus for steam hydrating alkali silicate glass and subsequently extruding the hydrated material as a foam extrusion. The treatment temperature although it can be up to 600.degree. C. will not generally exceed the softening point of the anhydrous glass.
The above-noted disclosure either taken together or individually do not disclose a thermal insulating product of an expanded, shell-less silica-hydrate. The foamed or expanded silica-hydrate is produced by the selective heating of silica-hydrate particles in a moisture enhanced environment by radiated energy at a predetermined frequency or may be accomplished in a media at a specific temperature range below the melting or liquid state of the silica-hydrate.